Intermittent Rough Running Diagnosis

**tony-baroni** · 10-17-2019

Finding the root cause continues to elude me...let me share what I've done in a couple checks this evening and last.

Replaced the TBI base gasket, torqued to specs 12 ft lbs.
Verified the hose to the MAP holds vacuum
Verified the hose and vacuum canister hold pressure (though they slowly leak, not enough to make this idle so high)
Verified the PCV valve and hose don't have any cracks, and is in place
Verified the hose from the TBI to the EGR control unit is in place.
Verified the hose from the EGR to the EGR control unit holds vacuum

Found that the charcoal canister hose, that runs from a port on the front of the TBI, does NOT hold vacuum. I do not know if it is supposed to!

Then I tried setting the IAC.
Warmed engine up and in closed loop mode.
Turned key off, jumped the 1 & 2 terminals on the ALDL port, put key to ON for ten seconds. I have read the IAC should be buzzing at this point, but there was no noise. Unplugged IAC. Key off, jumper out. Plugged in PC, and started vehicle, IAC showed zero, and idle was still 1900. Is it possible my IAC is not working and holding it wide open? Any other checks I should try?

Pressed on the underside of the EGR valve, which dropped the idle while I held the valve open. So that doesn't appear to be a leak...

I also tried using Wd40 spray on the ports and intake gaskets but it didn't seem to have any effect. I then tried spraying it directly into the TBI, and it didn't have any effect there either. The fan is so strong at 2000 rpms that it's just dispersing the wd40 too quickly.

**stew86MCSS396** · 10-17-2019

That's because that procedure to adjust the minimum air doesn't work for PCMs. Middle of post #9 mentions how to adjust it.

**tony-baroni** · 10-17-2019

Good morning all. Here's a recent log file, after I swapped the memcal. When running with this one, I'm not seeing the massive number of knocks. I didn't drive it, just was idling.

IAC shows zero at idle, which I think means it's fully closed based on the fact the idle speed is so high.
BLM and INT both are below 128, which I believe means the engine is running rich and the PCM is trying to pull fuel out. BLM goes as low as 96, while INT is rising back up towards 128.

This is a bit counter intuitive. I've been assuming that the idle is high due to a vacuum leak, which would indicate a lean condition, and the system would be adding fuel. The PCM should know it's supposed to be at idle since the TPS shows a zero or near zero throttle position. What do you all think about that?

Re: IAC
I don't follow the steps in post 9 to set IAC. It says to adjust the idle screw until you see a certain count, but that's already backed off all the way. So opening the throttle blades further would increase the engine speed further, and keep the IAC at zero, right?
It also talks about turning off the key for 10s, which I do every time I stop troubleshooting. So according to my interpretation, my IAC should be set. I'm just uncertain....and trying to confirm everything.

I'm going to have to take a break as I'm headed out of town tomorrow AM. I might be able to read the stock chip and post it here tonight as I ordered a part from Moates to allow me to do that.

**tony-baroni** · 10-22-2019

Tonight I did my best to confirm no vacuum leak. Sprayed starting fluid everywhere...no indication of any leak.

Did the battery off reset of IAC.

I confirmed it appears my vehicle doesn't have a solonoid valve on the EVAP canister, so that acts lke a constant vacuum leak....but I confirmed with a short plugged off vacuum line that it had no effect anyways on the idle.

I don't know what to check next, so unless the group has an idea it's going to have to go to my mechanic..what a bummer! I hope he can use a scan tool to confrm the IAC is responding. Perhaps it's stuck wide open and that's causing the problem...

**stew86MCSS396** · 10-22-2019

Watching one of your logs^^^10-13-19 your IAC almost never ventures from 0. Depending on what's programmed into the PCM typically 145, Key On Engine Off, then 145 is what you should see and you should be able to see this in TPRT. IDK...going back and reading your 60 posts...I'd be suspect of the SPR bored out TBI you have. Was that ever changed to perhaps a stock unit? It appears that this engine should run on a stock tune based on the information that you provided.

**tony-baroni** · 10-22-2019

I sent in my TBI to SPR as a core, and he transferred the throttle brackets over, so I haven't tried a stock one on this engine. I do not have a spare. I agree that I didn't need to perform that upgrade....perhaps I will try a stock burn chip. I was hoping to get it back to running as well as possible with the current chip and setup, before changing things further.

**tony-baroni** · 10-23-2019

I burned a chip with the stock bin, and validated it with Moates burn2. Chip is from Moates.

Installed it, and the thing started up. Set the base timing back to zero.
Went through the IAC reset procedure.

The service engine light is now flashing on/off just over once per second. TP5 shows an error at the same rate in the bottom status bar. It also doesn't show many of the sensor values, and shows every error code has been set.

I removed the chip and adaptor, reset it to make sure it was seated properly, tried again, same result.

I will have to do some more searching to see what might be causing this one...